tu

The Latin verb ending already tells you who is acting, so amas on its own is "you love" and the pronoun is optional. When tu does appear it is because the speaker wants emphasis or a contrast: tu venis, ego maneo. The forms worth memorising are the nominative tu, the accusative and ablative te, the dative tibi and the genitive tui; the plural is vos, vobis. With cum the ablative attaches as an ending, so it is tecum, never cum te.